Receptors, in general, are molecular structures located in the cell membrane or within a cell that form a weak, reversible bond with an agent such as an antigen, hormone, or neurotransmitter. Each receptor is designed to bind with a specific agent(s). A specific family of receptors is the seven transmembrane (“7TM”) or G-Protein-Coupled Receptor (“GPCR”). These receptors link with a Guanine Nucleotide-Binding G-protein (“G-protein”) in order to signal when an appropriate agent has bound the receptor. When the G-protein binds with Guanine DiPhosphate (“GDP”), the G-protein is inactive, or in an “off position.” Likewise, when the G-protein binds with Guanine TriPhosphate (“GTP”), the G-protein is active, or in an “on position” whereby activation of a biological response in a cell is mediated.
GPCRs share a common structural motif. All these receptors have seven sequences of between 22 to 24 hydrophobic amino acids that form seven alpha helices, each of which spans the membrane (i.e., transmembrane-1 (TM-1), transmembrane-2 (TM-2), etc.). The transmembrane helices are joined by strands of amino acids between transmembrane-2 and transmembrane-3, transmembrane-4 and transmembrane-5, and transmembrane-6 and transmembrane-7 on the exterior, or “extracellular” side, of the cell membrane (these are referred to as “extracellular loops” or “extracellular” regions). The transmembrane helices are also joined by strands of amino acids between transmembrane-1 and transmembrane-2, transmembrane-3 and transmembrane-4, and transmembrane-5 and transmembrane-6 on the interior, or “intracellular” side, of the cell membrane (these are referred to as “intracellular loops” or “intracellular” regions). The “carboxy” (“C”) terminus of the receptor lies in the intracellular space within the cell, and the “amino” (“N”) terminus of the receptor lies in the extracellular space outside of the cell.
Generally, when a ligand binds with the receptor and “activates” the receptor, there is a change in the conformation of the intracellular region that allows for coupling between the intracellular region and an intracellular “G-protein.” It has been reported that GPCRs are “promiscuous” with respect to G-proteins, i.e., that a GPCR can interact with more than one G-protein (Kenakin, 1988). Although other G-proteins exist, currently, Gq, Gs, Gi, and Go are G-proteins that have been identified. Ligand-activated GPCR coupling with the G-protein begins a signaling cascade process or signal transduction. Under normal conditions, signal transduction ultimately results in cellular activation or cellular inhibition. It is thought that the third intracellular loop (IC-3) as well as the carboxy terminus of the receptor interact with the G-protein.
In general, the activity of almost every cell in the body is regulated by extracellular signals. A number of physiological events in humans as well as with a wide range of organisms use protein mediated transmembrane signaling via GPCRs. Signals from a specific GPCR cause activation of a G-protein in the cell. The majority of signals are transmitted by means of GPCRs into the cell interior. There are many varying aspects of this signaling process involving multiple receptor subtypes for GPCRs and their G-protein linked counterparts as well as a variety of linked intracellular secondary messengers. The signal transduction may result in an overall or partial activation or inactivation of an intracellular process or processes depending upon the proteins that are involved. Important signaling molecules or neurotransmitters which bind to GPCRs include, but are not limited to corticotropin releasing factor, parathyroid hormone, morphine, dopamine, histamine, 5-hydroxytrytamine, adenosine, calcitonin, gastric inhibitory peptide (GIP), glucagon, growth hormone-releasing hormone (GHRH), parathyroid hormone (PTH), PACAP, secretin, vasoactive intestinal polypeptide (VIP), diuretic hormone, EMR1, latrophilin, brain-specific angiogenesis inhibitor (BAI), cadherin, EGF, LAG, (CELSR), and other similar proteins or molecules.
GPCRs constitute a superfamily of proteins. There are currently over 2000 GPCRs reported in literature, which are divided into at least three families: rhodopsin-like family (family A), the calcitonin receptors (family B), and metabotropic glutamate family (family C) (Ji et al., 1998). The reported GPCRs include both characterized receptors and orphan receptors for which ligands have not yet been identified. (Wilson et al., 1999; Wilson et al., 1998; Marchese et al., 1999). Despite the large number of GPCRs, generally, each GPCR share a similar molecular structure. Each GPCR comprises a string of amino acid residues of various lengths. GPCRs lie within the cell membrane in seven distinct coils called transmembranes. The amino terminus of the GPCR lies outside the cell with the extracellular loops, while the carboxy-terminus lies inside the cell with the intracellular loops.
The ligands for GPCRs comprise small molecules as well as peptides and small proteins. The interactions between these ligands and their receptors vary from system to system but they may require the interaction with residues in several of the four extracellular domains and the N-terminus. In some instances the N-terminus alone may maintain an ability to interact with or bind ligands. GPCRs with known ligands have been associated with many diseases including multiple sclerosis, diabetes, rheumatoid arthritis, asthma, allergies, inflammatory bowel disease, several cancers, thyroid disorders, heart disease, retinitis pigmentosa, obesity, neurological disorders, osteoporosis, Human Immunodeficiency Virus (“HIV”) infection and Acquired Immune Deficiency Syndrome (“AIDS”) (Murphy et al., 2000; Mannstadt et al., 1999; Berger et al., 1999; Jacobson et al., 1997; Meij, 1996;).
Accordingly, there is a need in the art for methods of producing modulators of GPCRs and the ligands that bind GPCRs for use as therapeutics. These therapeutics may be used to prevent or treat GPCR associated diseases and/or disorders.